There are a number of medical conditions for which it has been found that an effective therapy involves driving current through a section of the tissue of a patient. Often, the current is driven between the electrodes of an electrode array implanted in the patient. Generally, the electrode array includes a non-conductive carrier on which typically two or more electrodes are disposed. Once the electrode array is implanted, current is driven from at least one of the electrodes, through the adjacent tissue, to at least one of the other electrodes. The current flow through the tissue influences the tissue to accomplish a desired therapeutic result. For example, an electrode array positioned adjacent the heart may flow currents to stimulate the appropriate contraction and expansion of the heart muscles.
There is an increasing interest in implanting electrode arrays adjacent neural tissue so that the resultant current flow induces a desired neurological or physical effect. In one known application, the current driven between the electrodes of an array placed on top of the dura in the vertebral column reduces the extent to which chronic pain signals are perceived by the brain. Alternatively, the array may be placed in a location where the current flow stimulates a feeling of satiation as part of an appetite suppression/weight management therapy. In another application, the current is flowed to tissue or nerves associated with the bladder or the anal sphincter to assist in control of incontinence. Electrodes may be implanted in a paralysis victim to provide muscle control and/or a sense of feeling.
The Applicants' PCT Patent Application No. PCT/US2009/33769, FOLDABLE, IMPLANTABLE ELECTRODE ARRAY ASSEMBLY AND TOOL FOR IMPLANTING SAME, filed 11 Feb. 2009, published as WO 2009/11142, the contents of which are explicitly incorporated herein by reference, describes an electrode array that includes a carrier on which plural electrodes are arranged in a row by column matrix. An advantage of this electrode array is that it allows current to be flowed between numerous different combinations of electrodes. Depending on which electrodes are connected to associated current sources and sinks, this array can be operated so that there are two or more current flows occurring simultaneously between different sets of electrodes. Once this assembly is deployed, the practitioner can initially drive current between different combinations of electrodes. Current therefore flows through different sections of tissue. This allows the practitioner to determine between which electrodes, through which tissue, the current flow offers the greatest benefit and/or tolerable side effects. Once the optimal current flow path between the electrodes is determined, the array and its associated power supply are set to operate in this state.
The Applicants' PCT Patent Application METHOD OF ASSEMBLING AN ELECTRODE ARRAY THAT INCLUDES A PLASTICALLY DEFORMABLE CARRIER, filed 29 May 2009, published as PCT Pub. No. WO 2009/155084, the contents of which are explicitly incorporated herein by reference, describes a means of batch assembling the above-described electrode array.
In comparison to other electrode arrays with lesser numbers of electrodes, the above-described array makes it possible to flow current through more sections of tissue and to selectively focus/diffuse the current flow. In contrast to an electrode array with a smaller number of electrodes, use of the above-described array increases the likelihood that the current flow can be set to provide desired therapeutic effects, with tolerable side effects.
Still another advantage of the above-described array is that the carrier is formed from superelastic material. A superelastic material is one that, after being subjected to appreciable bending or folding, returns to its initial state. Thus, once this electrode array is formed, the assembly is then folded or rolled into a form that has a side-to-side width appreciably less than its width in the unfolded/unrolled state. A benefit of an electrode array assembly of this design is that it can be folded into a sheath. The sheath-encased electrode array assembly can then be inserted through an access cannula using a minimally invasive procedure into the patient. Once in the patient, the sheath and assembly are steered to over the tissue against which the electrodes integral with the assembly are deployed. Once the assembly is properly positioned, the sheath is opened up or removed. The opening/removal of the sheath causes the carrier to unfold. As a consequence of the carrier unfolding, the electrodes deploy over the target tissue. A more complete understanding of how the electrode array assembly can be so positioned and deployed is contained in the Applicants' Assignee's PCT App. No. PCT/US2010/029628, DELIVERY ASSEMBLY FOR PERCUTANEOUSLY DELIVERING AND DEPLOYING AN ELECTRODE ARRAY AT A TARGET LOCATION, THE ASSEMBLY CAPABLE OF STEERING THE ELECTRODE ARRAY TO THE TARGET LOCATION, which is explicitly incorporated herein by reference the contents of which are published in US Pat. Pub. No. US 2012/0022551 A1.
Thus, not only does an electrode array built on a superelastic carrier provide a means for selectively flowing current through different sections of tissue, the assembly can be placed over the target tissue without having to cut a large incision in the patient.
One feature of the above-described array is that also mounted to the carrier are one or more drive modules. The drive modules contain the components that source/sink the current to/from the electrodes. It is necessary to provide some on array control circuitry because the array typically includes 10 or more and often 20 or more electrodes each of which serve as a current source and/or sink. Without providing these modules, it would be necessary to implant a large number of conductors that extend from the pulse generator, through the patient, over which the current is sourced/sunk to the individual electrodes. Physical constraints make it difficult to implant large numbers of conductors in the patient. The above referenced applications described how an electrode array may be constructed so that the drive module is disposed on the surface of the array on which the electrodes are disposed; the surface of the assembly disposed against the tissue. Alternatively, the drive module may be positioned on the surface of the carrier opposite the surface that faces the tissue.
Regardless of the location on the surface of the carrier on which the drive module is located, it is necessary to encase the module in some sort of package. The package protects the semi-conductor die forming the drive module. Often the package includes a shell and a cap. The shell surrounds one end and the sides of the die. The cap covers the exposed end of the array and the perimeter of the shell. Consequently, the known assemblies have some sort of conductors that extend from the electrodes, through the package to the semiconductor die. As mentioned above, a significant feature of the known assembly is that the carrier has some degree of flexibility. Accordingly, the conductors disposed on the carrier of this assembly are subjected to some flexing. Inside the package, the conductors are held rigid. Accordingly around the perimeter of the package, where the conductors are stopped from flexing, the conductors may be subjected to considerable stress. There is a concern that this stress could induce failure in the conductors.
Moreover, inside the package, wire bonds may have to be used to establish the final connections between the conductors and the complementary bond pads on the control module-forming semiconductor die. These wire bonds, given the fragility of the wires from which they are formed, may also be prone to breakage. In regard to this matter is should be appreciated that once electrode array assembly is implanted, the assembly, like the patient in which it is implanted, is almost always moving. Over time, the vibration induced by this movement can potentially cause these wire bonds to fracture. Clearly, the failure of these wire bonds, or complementary conductors can result in malfunction of electrode array.